I would say "Pennies have never flown upwards any time I dropped them. They have fallen downwards every single time. I will bet you one penny that it will fall downwards the next time. Then two pennies the time after, then four the time after that. I am confident enough in the laws of physics to put my money where my mouth is - are you as confident in your god?"
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'